A 52-year-old man with type 2 diabetes mellitus presents to the nephrology clinic for evaluation of progressive renal dysfunction. His serum creatinine is 2.8 mg/dL (baseline 1.0 mg/dL 2 years ago), and 24-hour urine protein is 2.5 g/day. His blood pressure is 158/94 mmHg. Urinalysis shows 3+ proteinuria and occasional hyaline casts. His eGFR (by MDRD equation) is 28 mL/min/1.73 m². What is the most appropriate next step in management?